110/220V Power System

Nice!

110/220V is not 3P.

110/220 is a 2-phase system. The 2 phases are 180 degrees apart. So when one phase is at +110 VRMS with respect to neutral, the other is at -110 VRMS, and the phase-to-phase voltage is 220 VRMS. This is the system common in most USA homes.

Correction, it is NOT 2 phase. it is Single Phase. it is not 2 phases that are 180 degrees out, it is two ends of the same transformer winding. The 180 degree difference is just opposite ends of the same phase.

2 phase power is something entirely different and now very rare. It takes 4 wires to make 2 phase power.

This is Good and TRUE.

mm

As others have already pointed out, 110/220V 60Hz is not a 3 phase voltage. The 3 phase equivalent is 120-208Y whereas 208 is the phase-to-phase voltage and 120V is derived from phase to neutral.

Sir,

You said: This connection point is sometimes called the "wild leg" or "red leg". Conductors of center-tapped delta systems connected to this point are often color coded with red insulation (for 220, 240 V center tapped delta systems). If you get curious and measure between the "Wild" leg and the center tap you will get a reading of around 219V but this tends to wander a bit depending on how the loads on the legs are (or are not) balanced.

I think there is an error in your mathematics. For a 3-phase 4-wire center-tapped delta system as you describe, the "wild" or "high" leg on a 220V system will be 190V, and on a 240V system it will be 207V. As to its wandering a bit, this is true, but mostly because of the transformer being open-delta or there being different voltage drops on the conductors because of imbalanced loads. (Load balancing on a wild leg delta system is impossible, because line-neutral loads are always restricted to two of the three phase conductors, as you correctly state.) Also, in the USA, the National Electrical Code requires that the insulation color code for the wild leg be orange; I don't know about red in other countries.

In the next paragraph, you said: the square root of three never comes into play in developing or calculating the voltages. Actually, the square root of three is directly involved in calculating voltages in a 3-phase wye system or a 3-phase center-tapped delta system. That is because in a 30-60-90 degree triangle, with a hypotenuse length of 2, the two legs have lengths of 1 and √3.

--JMM

OK, lets try to clear so many unclear info above:

First: In USA Power Source voltages are 105% of Load voltages. So simple home appliances - single phase are rated 115 V for 120 V supply, 230V for 240 V supply.

Second: two-phase systems are rare. Their phases are 90 degree not 180. 180 degree is measured in Center Taped Transformers 120V Hot - Neutral (N) - 120 V if your oscilloscope Common is connected to N and each of two probes are connected to Hot 120 V. This does not means you have two - phase system! Take EET -101 class in the Tech College.

Third: In most states they have three-phase systems with line voltages (between two "hots") 120 , 240 or 480 V 60 Hz. Most popular 3-phase systems are 240/138 and 480/277 V. You may see also 208/120 V.

Fourth: Three phase systems are majority of Middle Voltage and High Voltage AC (alternating current 60 Hz) systems. The rated voltages have written line voltages (between any of two Hots from three hot conductors). Power from High voltage transmission lines (overhead on poles or towers as well as HV cables) to be used in low voltage installations up to 600 V ac, must use transformers. Three single-phase or one three-phase construction. They have interesting connections (Delta, Wye, zigzag) on both sides (High Voltage winding - low voltage windings).

Fifth: Relation between line and phase (to neutral) voltages is square root of 3 = 1.73 as the initiator of this discussion brought from standard European low valtage 50 Hz systems 380/220 V (load) or 231/400 V (power source - secondary transformer winding voltages). You may see also 660/380 V systems there.

Friends,

I believe that some answers have assumed the "3P" statement to be 3-phase instead of the intended 3-pole.

In the USA the 110/220V 3P system is derived from a single transformer with primary connected to two phases of the 3-phase distribution system and secondary having a grounded center tap. This is a standard single-phase transformer, supplying a single-phase 3-wire load.

In the described 220/380V 3P system, power is again derived from two phases of the 3-phase distribution system, but with two separate transformers (or two separate windings in a single case). The secondary neutral is actually the middle of a wye system, with only two legs. Thus the phase angle between the two line conductors is 120-degrees (instead of 180-degrees with the single-phase system). The voltages involved are 220V between either line and neutral and 380V between the two lines.

I have not traveled in countries where this type of system is common, so I don't know how the distribution is handled. I suspect that the distribution secondary is actually 3-phase 4-wire and that only two phases and the neutral are brought to each residential structure.

--JMM

Interesting forum. Here is another two cents worth. In most of the world three-phase power is delivered to buildings using only a WYE (Or STAR) transformer. Three-phase voltages are nominal 380, 400, or 415. A connection to the center tap creates a single-phase voltage that is about 57% of three-phase voltage (Nominal 220, 230, 240 volts).

American style electrical systems (Some exist outside the America) have more variations--and cause more confusion.

(1) Three-phase power is delivered at either 460, 230, or 208 volts (480 and 240 are basically the same as 460 and 230--as mentioned above, there are regional differences).

(2) Single-phase power is either nominal 115 (110-120) volts, or nominal 230V (220-240) volts.

(3) The three-phase transformers (Often three single-phase transformers), can be wired in three different methods: (a) WYE or STAR with a center tap, (b) Closed delta, or (c) Open Delta (Using only two transformers).

(4) All three arrangements can deliver reliable three-phase power. By using only two of the three terminals, all three can deliver single-phase power at the same voltage. However only two of them can deliver single-phase power at a different voltage.

(5) A 460V or 230V WYE transformer delivers an unusable (57%) single-phase voltage. However, a 208V WYE transformer delivers a usable 120V between all three legs and the center tap. These are often used to supply both three-phase and single-phase power to a small commercial building.

(6) A 230V Closed Delta can have a center tap (May be called Neutral) between two terminals (Say L1 and L2), and deliver 115V single-phase power between L1 and Neutral, and also L2 and Neutral. A connection from L3 to Neutral creates an unusable "Wild Leg" voltage of about 190 volts.

(7) An open delta can actually deliver three-phase power using only two single-phase transformers (Impressive!). Single phase power is available, if you chose to connect to only two of the terminals, however, the single-phase volltage will be the same as the three-phase voltage (Nominal 460 or 230).

(8) As mentioned above, most residences receive nominal 230V single-phase only. By wiring the transformer L1---N---L2, the customer can get 230V (L1 to L2) for large applicances, and 115V (L1 to N, and L2 to N) for small appliances. Whew!

Thank you for your organized Intro!

A few comments & clarifications:

American style electrical systems (Some exist outside the America) have more variations--and cause more confusion.

(1) Three-phase power is delivered at either 460, 230, or 208 volts (480 and 240 are basically the same as 460 and 230--as mentioned above, there are regional differences).

The differences are not because of "regional" . In the same electrical distribution system we have e.g. 240 and 230 V. First value is for power supply units/elements as transformers (they have on their Nameplates .../240 V, and second value is on the load (appliances, motors) nameplates. Again it is most often net/grid assumption that the existing drop voltage should not exceed 5 %. Read a basic books about feeders + circuits designing.

(2) Single-phase power is either nominal 115 (110-120) volts, or nominal 230V (220-240) volts.

These values apply to loads, e.g. lights (115V) or greater loads as oven/ranges , air conditioners (230 V)

(3) The three-phase transformers (Often three single-phase transformers), can be wired in three different methods: (a) WYE or STAR with a center tap, (b) Closed delta, or (c) Open Delta (Using only two transformers).

Transformer's windings are connected in three possible ways: Wye/Star, Delta or Zigzag, Three phase feeders (from transformers to Load Centers) are wired using three or (more popular) four wire systems (three "hot voltage" conductors + one neutral/grounded or grounding). One of the reason residential transformers are single-phase is that Primary terminals (H1 & H2) are connected directly to "hot" phases of overhead middle voltage lines, and secondary windings have middle (center) connection to N (neutral usually grounded) and 120 V "hots" are connected inside to x1 and x2 terminals. Service (drop to house) has three wires (sometime multi-conductors). Entering Fuse/Circuit Breakers Center / also Load Centers terminals are marked L1-N-L2. This solution leads to more single-phase motors produced in the USA comparing to Europe where more Load Centers are 3-phase.

By the way in Europe terminals are marked: U-V-W / X-Y-Z ("hot" = phase) and O and N (neutral/earth "zero")

(4) All three arrangements can deliver reliable three-phase power. By using only two of the three terminals, all three can deliver single-phase power at the same voltage. However only two of them can deliver single-phase power at a different voltage.

Even the description is correct, the better way to educate readers is to use diagrams. Of course better understanding diagrams require at least taking any of EET basic classes about Alternating Current _ Three Phase Systems chapter. E.g. why x-formers winding connections are described as "Delta", "Star/Wye", "Zig-zag"?. Electricians in USA must know so many different primary/ secondary windings connections and how then recognize them outside by using voltmeters on x-former terminals.

(5) A 460V or 230V WYE transformer delivers an unusable (57%) single-phase voltage. However, a 208V WYE transformer delivers a usable 120V between all three legs and the center tap. These are often used to supply both three-phase and single-phase power to a small commercial building.

Everybody understood?

(6) A 230V Closed Delta can have a center tap (May be called Neutral) between two terminals (Say L1 and L2), and deliver 115V single-phase power between L1 and Neutral, and also L2 and Neutral. A connection from L3 to Neutral creates an unusable "Wild Leg" voltage of about 190 volts.

From the basic definition of Supply Voltage you may not use the value of 230 V here. It should be 240V because closed delta apply in this case to the transformer (x-former - tech slang), right? Everybody knows what is L1-L2-L3-Neutral?

(7) An open delta can actually deliver three-phase power using only two single-phase transformers (Impressive!). Single phase power is available, if you chose to connect to only two of the terminals, however, the single-phase voltage will be the same as the three-phase voltage (Nominal 460 or 230).

Nominal (rather European term)/ Rated (USA term) value should be here 480 or 240

Open Delta delivers (or must be selected for power demand accordingly) two-third of three single x-formers connected in full Delta. If power supply is composed from three single-phase transformers it is possible to still deliver two-third of the rated power is the time when only two x-formers are transferring higher voltage system to low voltage distribution one

(8) As mentioned above, most residences receive nominal 230V single-phase only. By wiring the transformer L1---N---L2, the customer can get 230V (L1 to L2) for large appliances, and 115V (L1 to N, and L2 to N) for small appliances.

Again, check in Electricians Books: L1 (should have measured 120V to N) - N (should have almost zero volts to the Ground /Earth in UK) - L2(another 120 V). After 5% drop loads could have measured 115 or 230 V as stated above. By the way motors have terminals marked: T1-T2-T3.

Anyway, I hope my second step after first step explanation of USA systems from California, I did compared to East Coast and Europe (they also have some differences) system. In Europe and some places here transformers' secondary winding are connected is so called Zig-Zag. This configuration lets overcome some problems created by uneven power drawn from each phase.

Interesting topics is also "Harmonics". But leave it to these who will find time and appetite to read Electrician's Handbooks.

Southern123,

On item 5), the single-phase 277V of a 480/277V star (or wye) distribution system is commonly used in offices and commercial buildings for lighting. Consider the number of fixtures with 277V ballasts!

The NEC (National Electrical Code) in the USA does not limit the allowed voltage drop on feeders and branch circuits. The closest it comes to this is in §210-19(a) FPN No. 4, and in §215-2(d) FPN No.2. The FPN (fine print notes) are advisory, not mandatory. They indicate what is good practice, which is the suggested maximum of 5% drop from the service entrance to the furthest connected load (2% on feeders plus 3% on branch circuits). I believe that nearly all apprenticeship programs, regardless of sponsor or text, will include these suggestions as the basis for conductor sizing.

Also, the NEC does not specify the actual service voltages. The closest it comes to this is in defining the nominal voltages as 120, 240, 277, 480, 600. This definition has been in the NEC since at least 1978, but was not in its 1971 edition (I checked my copies; I don't have the one between them). The actual voltages will be supplied according to the utility company's service standards. Each company can be different. Depending on the time of year, the tap setting(s) on the distribution network, the distance from the transformer, and the load, I have read actual voltages from 107 to 134 volts on a residential service. It is true, as one person's post stated, that these nominal voltages have been slowly creeping upwards. (My 1971 code mentions 115/230 and 120/208 as circuit voltages in one section.) When I was a child, the voltages discussed were 110 and 220. Perhaps some electrical historian can give more information on this, but I suspect the original nominal voltages were probably 100, etc.

--John M.

Dear John

You are 100% right citing NEC. For anybody outside USA - National Electrical Code is the set of regulation (something must/shall be done according to....), has a lot of practical tips, tables, examples of calculations etc so it is a basic Electrician"s Handbook. Some paragraphs/articles are not obligatory as you did mention.

In contrast, German VDE and DIN are very strict sets of rules, do not have handbook character but they are roots for other EU countries National Code (like VDE is) or IEEE standards (DIN - even more strict rules).

Again in academic world (for technicians and engineers) there are standards / nominal definitions & values as rated voltages that rule practical world of application. In our topic: generated /system supply voltages and manufacturing standards for electrical loads. Here, it is worth to mention NEMA standards.

OK , we both almost begin to write a book. If you join this task, you must select its title!

Merry x-mas + Happy holidays

Dear John

You are 100% right citing NEC. For anybody outside USA - National Electrical Code is the set of regulation (something must/shall be done according to....), has a lot of practical tips, tables, examples of calculations etc so it is a basic Electrician"s Handbook. Some paragraphs/articles are not obligatory as you did mention. Somehow they are more liberal than these in UK. Of course it is my subjective point of view.

We in USA should remember that Great Britain has have very significant role as technical educator of the world as Germany is in EU.

In contrast, German VDE and DIN are very strict sets of rules, do not have handbook character but they are roots for other EU countries National Code (like VDE is) or IEEE standards (DIN - even more strict rules).

Again in academic world (for technicians and engineers) there are standards / nominal definitions & values as rated voltages that rule practical world of application. In our topic: generated /system supply voltages and manufacturing standards for electrical loads. Here, it is worth to mention NEMA standards.

OK , we both almost begin to write a book. If you join this task, you must select its title!

Merry x-mas + Happy holidays

Manufans,

You are welcome. I hope, on behalf of all of us at CR4, that the answers were given in a way which expanded your understanding without just sounding "smart" or "official". All of us are capable of teaching and learning.

--JMM